IMO you don't need to feed kitten food. Kitten food has more protein/calories for growing kitties, which is why adults may get fat if they're left to eat it and eat the same portion of kitten food as they would adult food.

YES, it is safe to feed adults kitten food, and kitten's adult food! You just need to adjust their portions accordingly!! Feed adults less of the kitten food, and kittens more of the adult food.

In most cases, age specific foods are just ways for food companies to earn extra money

In any case it seems you are going to have to feed the cats in separate rooms so the adults won't finish their portion and run and try to eat the rest of the kitten's!